1. Field of the Invention
The present invention relates to the oxidation of monohydroxy mono- and bicyclic aryl compounds, e.g., phenol, to form monomeric 1,4-quinones, e.g., p-benzoquinone.
2. Description of the Prior Art
The oxidation of disubstituted phenols by means of oxygen in the presence of copper salt complexes with amines, nitriles, or tertiary amides has been widely reported to give self-condensation products, i.e., polyphenylene ethers and/or diphenoquinones. A. S. Hay et al. (J. Am. Chem. Soc. 81, 6335-6 (1959)) described the oxidative coupling of 2,6-disubstituted phenols in an organic solvent containing an amine (pyridine) and a copper salt as a catalyst. With small alkyl substituents on the phenol, carbon-oxygen coupling occurred and linear polyphenylene ethers were obtained. With bulky substituents, carbon-carbon coupling occurred and the diphenoquinone was obtained. Hay later noted (J. Polymer Sci 58, 583 (1962)) that the oxidation of simple monohydric phenols had generally yielded very complex products unless the phenol had been substituted in both ortho positions. In the same paper (p. 585) Hay mentioned that when phenol itself was treated with oxygen in pyridine solution in the presence of copper (I) chloride, a complex tarry residue was formed.
In U.S. Pat. No. 3,306,874, Hay reported that primary and secondary aliphatic amines can be used in forming an amine-basic cupric salt complex for the oxidation of monohydric, monocyclic phenols to various self-condensation products providing the phenols are substituted in the 2- and 6-position. Another patent to Hay, U.S. Pat. No. 3,210,384, describes the oxidation of a 2,6-disubstituted phenol in the presence of a complex of a basic cupric salt and a nitrile or tertiary amide. 3,3',5,5'-Tetrasubstituted diphenoquinone was obtained.
Braxton et al. (U.S. Pat. No. 3,213,114) described oxidizing certain 2,4,6-trialkyl phenols (4 to 12 carbon alkyl groups) to 2,6-dialkyl-p-benzoquinones by means of oxygen and a catalyst, preferably a cuprous chloride complex of an amine. The patentees concluded that the presence of a hydrocarbon radical having a tertiary configuration in both positions ortho to the hydroxyl group as well as in the para position was necessary for the successful, practical oxidation of the phenol to the p-benzoquinone.
Japanese Application Publication 36,641/74, published Apr. 5, 1974, describes the oxidation of 2,3,6-trimethylphenol by means of oxygen in the presence of a cupric salt dissolved in a nitrile or tertiary amide to give chiefly trimethyl-p-benzoquinone when the molar ratio of copper salt to trimethylphenol is high, e.g., about 1/1, or a mixture of the benzoquinone with polyphenylene oxide at lower molar ratios.
German Pat. No. 2,221,624 (May 2, 1972) teaches that methyl-substituted phenols (mono-, di-, tri-, or tetramethyl) can be oxidized to the corresponding quinones by means of oxygen in the presence of copper and halogen ions provided that complexing agents are absent or limited. It is stated that especially when the phenol has only one or two methyl groups, the yield of monomeric quinone decreases as the amount of complexing agent increases.
The catalytic oxidation of phenol to p-benzoquinone by means of oxygen recently has been described in U.S. Pat. Nos. 3,859,317 and 3,870,731, both of common filing date and inventer. In the process of U.S. Pat. No. 3,859,317, a cobalt(II) or manganese(II) coordination catalyst is used in combination with an alkanol, benzonitrile, hexamethyl phosphoric triamide, N-alkyl-substituted amide, or sulfoxide solvent. In the process of U.S. Pat. No. 3,870,731, which is applied also to the oxidation of alkyl- or halo-substituted phenols, a copper(I), copper(II), or metallic copper catalyst and a thiocyanate, cyanate, cyanide, or halogen ion catalyst promoter are used in combination with a water, N-alkyl-substituted amide, alcohol, or sulfoxide solvent. In these systems the amount of phenol reacted generally is small even after long reaction times.
To summarize, copper complex cataysts appear to be superior with respect to level of activity, as evidenced by conversions achieved and reaction times required, in the oxidation of phenols by means of oxygen. However, the prior art teaches that, with these catalysts, complex products are obtained unless the phenol is substituted in the 2- and 6-positions, and that complex tarry residues are obtained with unsubstituted phenol. The prior art also teaches that, with these catalysts, 2,6-disubstituted phenols give self-condensation products, i.e., polyphenylene ethers and/or diphenoquinones. In copper complex catalyst systems, uncoupled products have been reported only in the case of the oxidation of trialkyl-substituted phenols, but large quantities of polyphenylene oxide also were obtained unless the molar ratio of copper salt to the phenol was high. In German Pat. No. 2,221,624, complexing agents were described as being inimical to the formation of monomeric quinones in oxidations of mono-, di-, tri-, or tetramethyl phenols in the presence of copper and halogen ions, and such agents were described as preferably being absent, or at least present only in limited amounts.
1,4-Quinones are desirable as intermediates to hydroquinones, which find utility as antioxidants, reducing agents, polymer intermediates, etc.